Electronic components are attached to printed circuit boards (PCBs) by soldered connections. When attaching or detaching electronic components, the solder connection to the PCB is heated to a temperature at which solder reflow occurs. However, performing a proper solder reflow operation is a much more complex task than simply heating up the solder to its reflow temperature. This is due to the fact that the temperature profile of the solder needs to be maintained within an appropriate narrow process window that changes over time. In other words, the temperature of the solder connecting the electronic component to the PCB must be maintained within an appropriate narrow temperature range, with this narrow temperature range being varied over time to achieve proper solder reflow conditions. Moreover, there are temperature considerations when dealing with the electronic component and the circuit board itself. For example, excess temperature differentials through the electronic component may tend to damage the electronic component. Damage can also be caused by simply subjecting the electronic component to excessively high temperatures for an extended period of time. It is also necessary to avoid excessive temperature differentials across the PCB to avoid causing warpage of the PCB itself. As such, there are numerous reasons that the working temperature ranges of the solder, the electronic component and the PCB must all be maintained within clearly defined limits.
During a typical assembly or re-work operation, the temperature of the solder is typically increased in a series of controlled steps or stages, with each stage accomplishing a particular function in the overall soldering reflow process. The first stage is simply applying a low temperature “preheat”. This preheating stage removes any excess moisture from the PCB and the electronic component. Next, the temperature of the PCBA is raised during a “soak” stage at which time the temperature of the PCB is substantially equalized. Next, the temperature is raised during a “ramp” stage (resulting in rapid heating of the soldered connection, and activation of flux). Thereafter, the temperature is briefly stepped up to the actual “reflow” temperature stage. A cool down stage follows quickly thereafter.
During this process, it is important that the duration of time at which the solder temperature is actually in its “reflow” stage is not too long. This is due to the fact that such high temperatures (especially if prolonged) may tend to damage the electronic component itself. As a result, it is desired to raise the solder temperature just high enough to cause reflow, but only for a short period of time. As can be seen, the overall soldering reflow process necessitates operating within narrow temperature windows over time.
The above discussed problems are even more complicated when using today's lead-free solders. This is principally due to the fact that lead free solders have higher reflow temperatures. Therefore, it is necessary to heat lead-free solders to higher temperatures to achieve reflow. Yet, it is still important that maximum temperatures (or temperature differentials) are not exceeded so that PCB becomes warped. Consequently, working with lead-free solders requires operating within a much tighter temperature profile window, and thus, a much tighter temperature management and control system is required.
Various systems have been devised to provide heating to the solder connecting the electronic component to the PCB. Unfortunately, these existing systems all tend to suffer from various disadvantages.
A first type of system is a simple forced air convection system. Examples of such forced air convection systems include Summit 1100 made by SRT, Inc. of Connecticut. In such systems, a pre-heater is positioned below the PCB to direct heated air upwards against the bottom of the PCB, thereby raising the temperature of the PCB above ambient temperature. A forced air convection heater in the tool head is then used to heat the electronic component from above. The bulk of the heat supplied to the solder is actually supplied from above (i.e. from the heater in the tool head). It is this heat from above that actually causes the solder to reflow.
Unfortunately, these type of systems have disadvantages. For example, problems exist when the forced air convection pre-heater is either too large or too small (as compared to the PCB positioned thereabove). Specifically, if the pre-heater is too large, the system will be very thermally inefficient, since a large portion of the heat is simply lost around the sides of the PCB. Moreover, it is very uncomfortable for an operator to work with such a system since the heat is simply directed upwards into the operator's face and hands. Conversely, if the pre-heater is too small, most of the thermal output will be focused at the center of the PCB. This causes the PCB to have an uneven temperature profile thereacross (i.e.: in its X- and Y-axes). Such non-uniform temperatures across the PCB may tend to cause the PCB to warp or to deform. Since the same pre-heater is used to work on different sized electronic components and on different PCB's, the operator is constantly attempting to deal with the problem of the pre-heater being either too large or too small for the job at hand.
A second type of system uses infa-red heating of the PCB. Examples of such infa-red heating systems include the 936 A system made by Fonton Inc. of Taiwan. Infa-Red heating has its own particular disadvantages. For example, it is slow in controlling temperature changes. This makes it especially difficult to achieve a desired temperature profile, especially when handling lead-free solders which demand narrow temperature windows during their various solder reflow heating stages. A further disadvantage of infa-red heating is that it produces questionable temperature uniformity in the PCB itself. This is due at least in part to the fact that different surfaces of the PCB have different absorption characteristics under infa-red heating.
Another existing system uses “hot plate” pre-heaters to heat the PCB. Examples of such systems include systems made by Airvac Corporation, of Connecticut. Hot plate heating uses a heated metal plate that is placed under a PCB assembly to transfer heat thereto by radiation or natural convection. Unfortunately, hot plate heading has the disadvantage of being very slow to respond to changes in the hot plate set-point temperature. Therefore, it is slow in controlling temperature changes. This makes it especially difficult to achieve a desired solder temperature profile, especially when handling lead-free solders which demand narrow solder temperature windows during their various reflow heating stages.
Legislation requiring lead free solder is becoming mandatory. Unfortunately, as stated above, lead-free solders have higher reflow temperatures. However, should the working temperatures applied to the electronic components become too high, the electronic components may be damaged. As a result, it is necessary to perform component assembly and re-work within very narrow temperature process windows during the various stages of the solder re-flow process.
A further disadvantage of all the above discussed systems is that the bulk of the heating that actually causes the solder to reflow is the heating that is applied downwardly from a heater disposed in the tool head. Thus, the soldered connections (which are found at the bottom of the component) are predominantly heated by heat that is applied to the top of the component (and conducted downwardly through the component). A disadvantage of this approach is that large, unwanted temperature differentials occur in the component's Z-axis. Specifically, the top of the component must be raised to a higher temperature than is necessary to cause the solder to reflow. The greater the temperature differential through the electronic component, the greater the possibility that the component will become damaged.
What is instead desired is a component assembly and re-work system that provides a minimal Z-axis (i.e.: vertical) temperature differential through the component, while still providing controllable heating of the solder connection within narrow time and temperature parameters. It is further desirable that such a system minimize the X-axis and Y-axis (i.e.: horizontal) temperature differentials across the PCB, so as to minimize the potential for warpage of the PCB.
As will be explained below, the present invention provides such a system, and offers numerous other advantages.